The present invention relates to a method for cleaning porous membranes, in particular microfilters that are used in an oil dewaxing process.
Oils, extracted from vegetable and animal tissues, contain a number of impurities, which have to be removed in order to make the oil suitable for human consumption. In the oil refining process several impurities can be removed by a filtration step.
The impurities in the context of the invention are substances and/or compounds that cause cloudiness in the oil upon cooling to a temperature range from ambient temperature to 0xc2x0 C. The impurities include proteins, glucosides, waxes, terpenes, squalenes, seed-residues, oxidized materials, or traces of earth.
One type of impurities consists of waxes, which are high melting esters of fatty alcohols and fatty acids, having a low solubility in oils. The quantity of waxes in crude oils varies from several hundred parts per million (ppm) to several thousand ppm. The wax content has to be reduced to a level of about 10 ppm or less, so that the cold stability of oil is improved. A known method for dewaxing oil consists of carefully cooling the oil, during which wax crystals are formed. This process is known as winterization.
Subsequently, the above indicated impurities can be removed by filtration. Several filtration methods are known from the prior art. The methods can differ in membrane material and type of filtration unit.
Known methods for filtration are filtration over filter cloth, paper or metal gauze filters at an average pore size of 25-100 xcexcm, filtration over fine filter cloth with an average pore size of 20-30 xcexcm, and filtration over microfilters with an average pore size of 0.05-5 xcexcm.
One known method which is particularly suitable for removing waxes and other impurities from oil, is filtration over microfiltration membranes. For instance, EP-A-397,233 discloses a dewaxing method in which after drying, the oil is cooled at a specific cooling rate, during which at a certain temperature wax crystals form. The solid wax is removed by microfiltration over a microfilter module having a filtration area of about 0,2 m2 and having a nominal pore diameter of 0.2 xcexcm. A second type of microfilter used in EP-A-397,233 is a sintered metal filter having a filtration area of about 0.18 m2 and a nominal pore diameter of 0.5 xcexcm.
A major drawback of all known filtering processes is that the wax and other impurities stick to the filters, thereby reducing the flow of oil through the filters. This flow through the filters is also called flux.
Flux will be used in this application as the amount of oil that can pass through a certain area of a membrane in a certain amount of time at a standard transmembrane pressure and at standard temperature. In practice, preferably a transmembrane pressure of 1-3 bar, more preferred 3 bar is used.
For the purpose of the invention, the side of the membrane at which the clean, filtered oil emerges is called the permeate side. The side at which the impurities accumulate on the membrane is the original liquid side.
The sticking of impurities to filters is also known as fouling. Several fouling mechanisms have been identified. Among these are: gel-layer fouling, adsorptive fouling and pore blocking.
Several methods are known to reduce fouling, especially pore blocking.
The most widely applied method to improve filtration over classical cloth or metal gauze filters is the addition of a filter aid to the oil that is to be filtered. This is described in Bailey""s industrial oil and fat products, fifth edition, vol 2, page 645, published by John Wiley and Sons, Inc. Drawbacks of this method are the high cost of the filter aid, extra oil losses and formation of solid waste.
For cleaning microfilters which are clogged by fouling several methods have been described. For instance, U.S. Pat. No. 5,482,633 discloses the use of ceramic microfilters in the wax-filtration process. A vegetable oil/particulate wax blend is filtered through a porous nonmetallic inorganic filter having a pore diameter of 0.5-5 xcexcm. Cross-flow filtration is applied. The described filtration process includes at least about one backflush with permeate for every period of cross-flow filtration of about 3 to 5 hours. The backflush takes a few seconds. After a designated period of use, the filter is regenerated with warm clean oil. It is mentioned that it is also possible to use a cleaning agent.
We have found that filters, cleaned by the process according to U.S. Pat. No. 5,482,633 will still show a high degree of fouling and relatively quick reduction of the flux rate in time. Another disadvantage is that these filters are expensive so regular replacement is not desirable.
DE-A-3312573 discloses a process for dewaxing of vegetable oil, under application of a porous membrane. Said membranes have a pore diameter of 0.05-xcexcm. In the course of time, when the flux is reduced a backwash treatment or rinsing treatment with a medium at 30-100xc2x0 C. is applied. The rinsing medium can for example be vegetable oil, n-hexane or acetone. In addition gases can be used, for example air, nitrogen, oxygen, carbon dioxide, argon or helium. We have found that the results of the cleaning process disclosed by this document leave to be desired.
JP 09/047641 discloses a method of washing a porous membrane. Oil is removed from a membrane used for removing wax from plant oil, and the membrane is subsequently subjected to a hydrophilizing operation and washed with water. According to the method disclosed in this document, after dewaxing and filtering the used membrane is subjected to reverse washing with gas and fluxing with oil. Then the membrane is rinsed with hexane followed by filtering to remove more oil. Subsequently a hydrophilizing treatment is carried out, whereupon a hot aqueous solution is passed through the membrane. Lastly the membrane is washed with water. Alternatively, methanol is passed through the membrane. According to the method disclosed in this document a treatment with hexane and methanol or ethanol is required. Applicants have found that the use of such organic solvents has several disadvantages. For example these compounds are hazardous to the health of the people working with them. Moreover the risk of fire or even explosions is considerable if such compounds are used. Furthermore the disposal of spent solvents having undesirable environmental effects may form a problem in industrial application of the methods disclosed in JP 09/047641.
JP 60/22906 discloses another process for cleaning filters wherein filter-clogging substances are removed from microfilters by applying a combination of flushing with an inert (compressed) gas and rinsing the filters with hot oil in which the waxes dissolve. In the wax-removing process, described in this specification, the wax accumulated on the surface of the membrane and probably also in the pores of the membrane, is subjected to a flushing (also called backwash) from the permeated liquid side of the membrane to the original liquid side of the membrane. As a result, the flux of the initial stage can be achieved again. The backflush is performed with compressed gas and/or dewaxed oil.
However, although gel-layer and adsorptive fouling may be reduced by the treatment disclosed in JP 60/22906, pore blocking can not be prevented and therefore micro-filters cleaned via this method have to be discarded and replaced regularly, for example about every 6 months. As these filters are very expensive, this filtration method is not widely used.
It has now surprisingly been found that it is possible to clean the microfilters in a special cleaning process by which the life time of the microfilters is extended, whereby the occurrence of pore blocking is effectively overcome. Furthermore, in this process the use of organic solvents is not required.
More in particular it has been surprisingly found that if the above mentioned filter recovery method comprising the steps of backflushing with inert (compressed) gas and backflushing with hot oil is combined with a regular treatment with alkaline material, preferably from the permeated liquid side to the original liquid side of the membrane, this leads to a very good recovery of flux through the membranes.
Accordingly the invention provides a method for cleaning porous membranes suitable for use in oil refining processes comprising the regular treatment of said membranes by applying the steps of:
(a) backblowing with gas
(b) backflushing with edible oil
(c) treating the membranes with alkaline material.